When Senate candidate Christine O'Donnell appeared on Sean Hannity's Fox News program, he read her some advice from another Fox commentator, Sarah Palin.

Palin, urging the Delaware Republican to "speak through Fox News," had said on Twitter that she should be spending her time with voters back home "versus appeasing national media seeking your destruction."

"She is absolutely right," said O'Donnell, although she has held almost no public events since that television appearance last month.

In the same period, her Democratic opponent, Chris Coons, appeared on MSNBC, where host Chris Matthews asked him about the inexperienced O'Donnell: "Does it bother you personally that someone like her, with that background, should run for public office?" Coons said no, but the leading question hung in the air.

The increasing polarization of cable news is transforming, and in some ways shrinking, the electoral landscape. What has emerged is a form of narrowcasting, allowing candidates a welcoming platform that helps them avoid hostile press questioning and, in some cases, minimize the slog and the slip-ups of retail campaigning.

"There's no question it's contributing to the splintering of the political system and the means by which people get information about that system," said Robert Thompson, who runs the Bleier Center for Television and Popular Culture at Syracuse University. "If there's no standard base line of fact and reporting, where can the conversation go?"

Each party's message is amplified by former officeholders and strategists who sign exclusive agreements with the cable networks as soon as they leave the public payroll. And their celebrity - magnified by their constant screen presence - gives them more influence than most members of Congress.

On any given night, a parade of Democratic lawmakers and candidates appears on MSNBC, while Republicans flock to Fox - including some, such as O'Donnell, Rep. Michele Bachmann (Minn.) and Sharron Angle, who grant few interviews elsewhere. CNN has a more balanced guest lineup and no openly partisan shows, but its prime-time ratings have declined substantially.

Angle, who is challenging Senate Majority Leader Harry M. Reid in Nevada, has said that the benefit of appearing on a "friendly press outlet" is that she can ask viewers for money: "When I said it on Sean Hannity's television show, we made $40,000 before we even got out of the studio in New York."

'Hit their base'

The reality, said MSNBC President Phil Griffin, is that "politicians want to hit their base." But "we're different than Fox," he added. "We ask for people to come on from both parties all the time. We can't control who comes on. A lot of people choose not to, and they choose to go to Fox. . . .

"We have so many different voices. We're not trying to push Democratic talking points, as some people accuse us of."

The White House has clearly chosen sides. President Obama recently called Fox News "destructive," renewing an attack that his administration launched against the network last year, while White House spokesman Bill Burton singled out MSNBC hosts Keith Olbermann and Rachel Maddow for helping to "keep our government honest." That praise followed a swipe that press secretary Robert Gibbs took at the "professional left," widely seen as aimed at liberal commentators and bloggers.